Heat exchanger

ABSTRACT

The invention relates to a plate heat exchanger ( 9 ), comprising a plurality of heat exchanger plates ( 1, 13 ), comprising at least one section showing indentations ( 2, 3, 14, 15 ), intended to be placed against corresponding indentations ( 2, 3, 14, 15 ) of a heat exchanger plate ( 1, 13 ) of a corresponding design. At least a first type of indentations ( 2, 14 ) and at least a second type of indentations ( 3, 15 ) is provided, wherein said first type of indentations ( 2, 14 ) and said second type of indentations ( 3, 15 ) are of a different design.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 11/662,218, filed Mar. 8, 2007, which application is a nationalization under 35 U.S.C. 371 of PCT/IB2005/053736, filed Sep. 7, 2005 and published as WO 2006/027761 A2, on Mar. 16, 2006, which claimed priority under 35 U.S.C. 119 to Sweden Application No. 0402152-3, filed Sep. 8, 2004; the benefit of priority of each of which is claimed hereby, and each of which is incorporated by reference herein in its entirety.

The invention relates to a plate heat exchanger, comprising at least one heat exchanger plate (preferably a plurality of heat exchanger plates) wherein at least one of said exchanger plates comprises at least one section showing indentations, intended to be placed against corresponding indentations of a heat exchanger plate of a corresponding design. Furthermore, the invention relates to a heat exchanger plate, comprising at least one section showing indentations, intended to be placed against corresponding indentations of a heat exchanger plate of a corresponding design.

Modern heat exchangers of the plate heat exchanger type are often provided with plates having a so-called herringbone pattern, i.e. a pattern which has indentations consisting of straight ridges and valleys. The ridges and valleys change their respective direction in the centre, producing the pattern that resembles a herringbone. In a stacked heat exchanger pack, alternate plates are turned by 180° so that the indentations cross one another. The thus stacked heat exchanger plates are brazed together, thus forming a compact and mechanically stable heat exchanger pack. Using the herringbone pattern of the heat exchanger plates, the resulting heat exchanger pack comprises a pattern of fluid channels through which the respective two fluids can flow and exchange their thermal energy.

When a heat exchanger pack of the afore-described type is exposed to pressure (in particular fluid pressure) and heat, the plates distort, causing a bending moment in the plates. In order to withstand high pressures, relatively thick metal sheets are used, e.g. with a thickness of 0.4 mm.

When such metal plates are pressed into the herringbone pattern, an unfavourable material flow takes place. If the press tool is not very accurately manufactured, cracks can appear in the plates. The relatively thick plates also require a high pressure in the press tool.

In a fully brazed heat exchanger, the joints are typically brazed with copper or a copper alloy solder placed between the plates. The copper (alloy) solder is frequently introduced as a coating of the metal sheets. The solder material collects at the crossing points of the indentations. The surface area and strength of the solderings are therefore quite small.

A fluid which is made to flow through a heat exchanger with a herringbone pattern is forced to flow over the ridges and down into the valleys. There are no unbroken straight flow-lines. At the leading edge of the ridges the flow rate is high, whereas the flow rate of the fluid is low behind the ridges (i.e. in the valleys). This variation in flow rate is very large. In the heat exchanger the heat transfer rate is high where the flow rate is high, but the heat transfer rate is low where the flow rate is low. A smaller variation in flow rate as it is the case in heat exchangers with a herringbone pattern is hence favourable.

When the flowing fluid contains two phases, i.e. the fluid is a mixture of a gas and a liquid, the recurring changes of direction at the ridges and valleys will have the effect that the gas forces the liquid away from contact with the plates. This reduction in wetting of the heat exchanger plates' surfaces also reduces the heat transfer rate.

The shape of the channels through a heat exchanger of the herringbone design also gives rise to a high pressure drop in the fluid as it passes through the heat exchanger. This pressure drop is proportional to the work done in forcing the fluid through the heat exchanger. A high pressure drop thus means high (mechanical) power consumption.

A heat exchanger trying to solve at least some of these problems is known from the document US 2007/0261829 A1. In this document it is suggested to provide a pattern on a heat exchanger plate that comprises indentations in the form of bulges and hollows, and between which channels are formed, passing through the heat exchanger. The shape of the thus formed channels gives rise to a moderate variation in flow rate through the heat exchanger, thereby resulting in a higher heat transfer rate. The thus formed heat exchanger plates are stacked together in a way that an upper plate is turned so that its downward-pointing hollows (bottoms) abut against the upward-pointing tops of a lower plate. The upper and lower plates are brazed together by forming solderings where the heat exchanger plates touch each other. However, it has been found, that these plates are prone to break in the side walls of the bulges during operation of the heat exchanger. Obviously, this seriously adversely affects the lifetime of the heat exchanger.

It is the object of the present invention to provide a plate heat exchanger that has improved characteristics over plate heat exchangers, known in the state of the art. It is another object of the present invention to provide a heat exchanger plate, in particular a heat exchanger plate for building a plate heat exchanger that has improved characteristics over heat exchanger plates, known in the state of the art.

It is suggested to design a plate heat exchanger, comprising at least one heat exchanger plate, preferably a plurality of heat exchanger plates, wherein at least one of said heat exchange plates comprises at least one section showing indentations and wherein said indentations are intended to be placed against corresponding indentations of a heat exchanger plate of a corresponding design in a way that at least a first type of indentations and at least a second type of indentations are provided, wherein said first type of indentations and said second type of indentations are of a different design. The expression “different design” can be understood in a broad way. The “different design” cannot only relate to the size and/or the shape of the respective indentation (especially when looking from above and/or from below onto the respective heat exchanger plate). For example, the different design (in particular the size and/or the shape) can relate to a cross-sectional view onto the respective structure, as well. Furthermore, even more different “designs” can be encompassed by this suggestion, for example a different thickness of the respective heat exchanger plate in the respective section, a different material, a different material coating, a different surface treatment or the like. Furthermore, when talking about an “indentation”, this does not necessarily mean that the respective section of the heat exchanger plate has been actively shaped. Instead, it is also possible that an indentation has been formed by actively shaping (for example by pressing or the like) of parts, being close to the respective indentation. Furthermore, the expression “indentation” can be understood in a very broad way, as well. As an example, an indentation can be a protrusion, a recess, a groove, a bulge, a hollow, a land, a web or the like. As it is usual with heat exchanger plates for plate heat exchangers, two plates, neighbouring each other, can be of an alternating, corresponding design. In other words, it is possible that a plate heat exchanger mainly consists of two differently arranged heat exchanger plates, having a corresponding design of indentations (wherein an indentation, going upward will contact an corresponding indentation from the corresponding heat exchanger plate that is going downward. Although it is in principle possible that two differently designed heat exchanger plates (or even more) are manufactured for building such a plate heat exchanger, for example, normally only a single heat exchanger plate is designed and manufactured, wherein the aforementioned two different “designs” of heat exchanger plates are achieved by turning every second plate in the stack of heat exchanger plates by 180°. Of course, the uppermost, as well as the lowermost plate has usually a different design for effectively closing the heat exchanger block. Typically essentially flat metal sheets can be used for this. After the stack of heat exchanger plates (and possibly other components) has been put together, the “raw” plate heat exchanger arrangement will usually be sent through a tunnel furnace to braze/solder the respective components together, to form a compact and mechanically stable block. Of course, it is possible that the plate heat exchanger will (essentially) show only the aforementioned two different types of indentations. However, it is also possible that a third, a fourth, a fifth or even more different types of indentations are provided as well. The presently suggested plate heat exchanger has to have (like any heat exchanger) two separate sets of fluid channels that are fluidly separated from each other. This is, because the thermal energy has to be transferred from one fluid to the other. In rare cases, more fluids, and hence more separated fluid channels, are used within a single heat exchanger. Usually, the two (or even more) fluids show different characteristics. For example, the two different fluids can have a different state of matter (for example, one fluid is a liquid, while another fluid is a gas). Also, one or both fluids can be a mixture of a gas and a liquid, with a varying gas to liquid ratio. Furthermore, the two different fluids will normally have a different temperature (at least at the entrance port of the stack type heat exchanger) and/or a different pressure. Even more, the different fluids can have a different viscosity, a different density, a different thermal capacity and so on. By forming the indentations with a different design, it is very easy to provide a mechanical stability that is different for the two different fluid channels, containing the two different fluids. This way, the mechanical stability of the plate heat exchanger can remain at the same level or can be even increased, while the overall dimension of the stack type heat exchanger can be reduced. Furthermore, using the proposed design, it is very easy to generate two different types of fluid channels for the two different fluids. As an example, the two different fluid channels can differ in cross section (in particular shape and/or size), the curvature of the respective fluid channel or the like.

In particular, it is possible that the plate heat exchanger is designed in a way that said first type of indentations and said second type of indentations are of a different size. Using such a design, it is particularly simple to provide different strength of the respective connections (for example to take into account different pressures of the respective fluids) and/or to adapt the sizes of the fluid channels, being formed between the respective connections, to the particular necessities of the respective fluid.

It can prove to be advantageous, if the plate heat exchanger is designed in a way that said first type of indentations and said second type of indentations show essentially the same shape. The “shape” of the respective indentation can be in particular the shape, when seen from above and/or from below onto the respective heat exchanger plate. Using the same shape can be particularly advantageous, if the respective shape has certain (advantageous) characteristics, for example a particularly low fluid resistance, a particularly high mechanical strength, a particularly advantageous ratio of surface area to the length of the surrounding edge or the like.

However, it can be also of advantage, if the plate heat exchanger is designed in a way that said first type of indentations and said second type of indentations are of a different shape. This suggestion is particularly useful if by choosing a different shape, the respective connections and/or the resulting fluid channels are particularly well suited for the specification of the respective fluid involved. As an example, by using a first shape for the first type of indentations, a very low fluid resistance can be achieved for the first fluid, used within the heat exchanger. By using a different shape for the second type of indentations, however, a higher fluid resistance can be achieved for the second fluid involved. Such a higher fluid resistance is introducing additional turbulence. Such additional turbulence can increase the possible heat transfer rate from the respective fluid to the channel wall and finally to the other fluid, thus utilising the higher resistance for increased heat transfer, thus increasing the performance of the resulting heat exchanger. In particular if a third, fourth (or even more) type of indentations is present, a mixture of “same shapes” and “different shapes” can prove to be useful, as well.

Another preferred embodiment of the plate heat exchanger can be achieved if the number of said first type of indentations and said second type of indentations are differing. Using this feature, it is also possible to adopt the strengths of the connections, the sizes of the respective resulting fluid channels and the fluid flow pattern within the respective fluid channels in a way that the result is particularly well-suited for the respective fluid. This way, an advantageous heat exchanger can be achieved.

In particular, it is possible to design the plate heat exchanger in a way that at least said first type of indentations and/or at least said second type of indentations show at least partially an elliptical shape, a circular shape, a teardrop-like shape, a polygonal shape and/or a symmetric polygonal shape. These shapes have proven to be particularly advantageous during first experiments. In particular, an elliptical shape and/or a circular shape usually result in a particularly high mechanical strength, a particular long lifetime of the resulting connection and/or a particularly large connection area, when compared to the bordering line of this connection area, combined with the relatively low fluid flow resistance. A teardrop-like shape will usually result in a particularly low fluid flow resistance, thus reducing mechanical energy losses.

Another preferred embodiment of a plate heat exchanger can be achieved if the number and/or the arrangement of at least said first type of indentations and/or at least said second type of indentations corresponds to the shape of at least said first type of indentations and/or at least said second type of indentations. By using such symmetries, a particularly strong heat exchanger with a long lifetime can be achieved, because mechanical stresses that are occurring are distributed comparatively homogeneously. Furthermore, using such symmetries, usually the resulting fluid flow patterns are advantageous, such decreasing fluid flow resistance and/or increasing heat transfer performance.

Another preferred design of the plate heat exchanger can be achieved if at least said first type of indentations and/or at least said second type of indentations are designed, at least in part, with an essentially flat top and/or bottom surface area. Having such a flat surface area, the strength of the resulting connection with the corresponding indentation of the neighbouring heat exchanger plate can be particularly strong, while soldering material (for example copper solder and/or copper alloy solder) can be saved.

Yet another preferred embodiment of the plate heat exchanger can be achieved if at least said first type of indentations and/or at least said second type of indentations are arranged, at least in part, along straight lines, wherein said straight lines are preferably arranged at an angle relative to a side edge of the corresponding heat exchanger plate. Using such an arrangement for the indentations, a simple, yet very efficient design of the heat exchanger plates can be achieved. In particular, it is possible that for building a complete plate heat exchanger, essentially only a single type of indented heat exchanger plate has to be used, whereas every second plate in the stack of heat exchanger plates is turned by 180° with respect to the respective neighbouring heat exchanger plates. This way, manufacturing tools and storage room can be saved, thus lowering production cost. The straight lines are preferably arranged at an angle of approximately 45° with respect to the corresponding side edge of the corresponding heat exchanger plate. However, certain variations around this preferred angle are possible. For example, the interval of possible angles can start at 30°, 35°, 40°, 42°, 43° and/or 44° and end at 46°, 47°, 48°, 50°, 55° and/or 60°. But the present invention in its broadest embodiment is not limited to any such angle.

Yet another preferred embodiment of a plate heat exchanger can be achieved if at least said first type of indentations and/or at least said second type of indentations are arranged, at least in part, in such a way that at least sectionally at least one of the circulating fluids has to follow a curved fluid path. This way, it is usually possible to increase the heat transfer rate of the respective fluid, thus increasing the performance of the heat exchanger.

Additionally or alternatively it is possible to design the plate heat exchanger in a way that at least said first type of indentations and/or at least said second type of indentations are arranged, at least in part, in such a way that at least sectionally at least one straight conduit for at least one of the circulating fluids is formed. By this design, the fluid flow resistivity can usually be decreased. This way, mechanical energy can be saved. This design is particularly useful with fluids, showing a particularly high and/or low viscosity and/or in combination with a design of the plate heat exchanger in which turbulence is generated by different means.

Furthermore it is suggested to design the plate heat exchanger in a way that at least said first type of indentations and/or at least said second type of indentations are arranged, at least in part, in such a way that at least sectionally at least one conduit for at least one of the circulating fluids is arranged in parallel to at least one of the side edges of the corresponding heat exchanger plate. This way, usually a particularly advantageous fluid flow between the fluid inlet duct and the fluid outlet duct of the respective fluid channel can be achieved.

Another particularly preferred embodiment of the plate heat exchanger can be achieved if at least one of said heat exchanger plates is formed, at least partially, of a metal plate and/or a metal alloy plate, wherein said plate preferably comprises, at least sectionally, a coating made out of an adhesive material, preferably made out of a soldering material. The metal plate can be, for example, made out of aluminum, an aluminum alloy, iron, copper, an iron alloy (for example steel), a copper alloy or the like. As an adhesive material, it is possible that a glue or the like is used. Of course, it is also possible that a soldering material (or brazing material) like copper or a copper alloy is used. It is to be noted that this suggested feature may be prosecuted in connection with the preamble of originally filed claim 1.

Furthermore, it is suggested that a heat exchanger plate, comprising at least one section showing indentations, that are intended to be placed against corresponding indentations of a heat exchanger plate of the corresponding design, is designed in a way that at least a first type of indentations and at least a second type of indentations are provided, wherein said first type of indentations and said second type of indentations are of a different design. Such a heat exchanger plate is particularly useful for manufacturing a plate heat exchanger of the above described type. Furthermore, the suggested heat exchanger plate can show the same features and advantages, as already described in connection with the stack type heat exchanger, at least in analogy. Furthermore, the heat exchanger plate can be modified in the aforementioned sense, at least in analogy.

The present invention and its advantages will become more apparent, when looking at the following description of possible embodiments of the invention, which will be described with reference to the accompanying figures, which are showing:

FIG. 1: a first embodiment of a heat exchanger plate for a plate heat exchanger in a schematic view from above;

FIG. 2: the heat exchanger plate of FIG. 1 in a schematic view from the side;

FIG. 3: a plurality of heat exchanger plates according to the embodiment of FIGS. 1 and 2, stacked together, in a schematic view from the side;

FIG. 4: a typical embodiment of a plate heat exchanger in a schematic perspective view;

Plate heat exchangers 9, such as the typical embodiment, shown in FIG. 4, are well-known devices for the transfer of heat between two different fluids. Plate heat exchangers 9 are used in many different applications, for example in the automotive industry, for cooling and heating of buildings and so on.

A plate heat exchanger 9 comprises a plurality of heat exchanger plates 1, 13 that are stacked over each other. The individual heat exchanger plates 1, 13 are designed with a pattern of indentations 2, 3, 14, 15, typically designed as bulges and hollows and/or as ridges and valleys (the latter one in particular in combination with the herringbone design). On the very top and the very bottom of the plate heat exchanger 9, flat metal sheets 16 are provided for retaining the fluids within the plate heat exchanger 9. Furthermore, connections 11, 12 for inlet 11 and outlet 12 of two fluids are provided as well.

The stack of heat exchanger plates 1, 13 is usually manufactured by loosely arranging the heat exchanger plates 1, 13 over each other and joining them together by soldering to form a mechanically stable integral unit.

Because of the pattern of indentations 2, 3, 14, 15 on the heat exchanger plates 1, 13, separate channels for the two fluids, are formed by the soldering process, wherein the separate channels are fluidly separated from each other. Typically, the two fluids circulate in a counterflow between alternate pairs of heat exchanger plates 1, 13. This technology as such is generally known.

FIG. 1 is a plan view onto a first possible embodiment of a heat exchanger plate 1, showing a distinct pattern of indentations 2, 3. As can be seen from FIG. 1, the depicted heat exchanger plate 1 is provided with a pattern of first bulges 2 and second bulges 3, and not with the currently widely used herringbone pattern. Furthermore, circular ports 17 are provided near the four corners of the heat exchanger plate 1. These circular ports 17 are the typical connections for the inlet 11 and outlet 12 of two different fluids into and out of the plate heat exchanger 9. Within the heat exchanger plate 1, shown in FIG. 1, a square is drawn with a dashed line. The respective surface part of the heat exchanger plate 1 is shown on the right side of FIG. 1 at an enlarged scale. Thanks to the enlarged scale, the pattern of first bulges 2 and second bulges 3 of the heat exchanger plate 1 is clearly visible. Both first bulges 2 and second bulges 3 are raised by a given height relative to a reference plate 18 in opposite directions. The flanks of the bulges 2, 3 have an edge angle of approximately 45 degrees. This deformation can be easily done by pressing techniques. In contrast to the herringbone pattern, the pattern of bulges 2, 3 of the present heat exchanger plate 1 is well suited to the pressing process, since the necessary deformation of the plate sheets is comparatively small. This way, the risk of cracks appearing in the heat exchanger plate 1 can be significantly reduced.

The first bulges 2 and second bulges 3 constitute a first pattern consisting of the first bulges 2, and a second pattern consisting of the second bulges 3. In the present embodiment of a heat exchanger plate 1, first bulges 2 and second bulges 3 have substantially flat first tops 4 and flat second tops 5 with a corresponding first surface area and second surface area, respectively. As can be seen from FIG. 1, the surface area of each individual first top 4 of the first bulges 2 is smaller as compared to the surface area of each individual second top 5 of the second bulges 3. Since the number of first bulges 2 and second bulges 3 is essentially the same, the overall surface area of the first tops 4 of the first bulges 2 is likewise smaller as compared to the overall surface area of the second tops 5 of the second bulges 3.

When a heat exchanger 9 is made from a plurality of heat exchanger plates 1, the heat exchanger plates 1 are connected such that e.g. the first surface areas 4 of one plate 1 are fixedly connected (soldered, brazed, glued) to the first surface areas 4 of a lower plate 1, and in the same manner, the second surface areas 5 of the one plate 1 are fixedly collected (soldered, brazed, glued) to the second surface areas 5 of an upper plate 1 (see, for example, FIG. 3). Due to the comparatively large surface areas of the first surface areas 4 and the second surface areas 5, relatively strong connections are made in the present embodiment. The connections by material engagement 10 are indicated in FIG. 3 between two neighbouring first surface areas 4 and two neighbouring second surface areas 5, respectively. The connection by material engagement 10 can be established by any process known in the art, such as brazing, soldering, glueing etc.

In operation, the heat exchanger 9 is filled with pressurised fluids (wherein the pressure of the two fluids involved can differ) which tends to force the heat exchanger plates 1 apart. The heat exchanger plates 1 can also expand due to increased temperatures, introduced by the fluids. Because of the pattern of first and second bulges 2, 3, all stresses generated in the plate material are directed essentially in the direction of the plate's material, and hence no or only small bending moments are created. The absence of such bending moments increases the strength and the lifetime of the structure. The strength of the heat exchanger 9 is also increased by the comparatively large contacting areas 10 between the first and second bulges 2, 3. Because of this improved strength, thinner sheet metal can be used for the heat exchanger plates 1. Alternatively, the sheet metal with the usual thickness of 0.4 mm can be used, giving the heat exchanger 9 a bursting pressure of 600 bar compared with 200 bar for a standard heat exchanger with a herringbone pattern and the same metal sheet thickness.

The heat exchanger 9 according to the present invention also offers the possibility that the opposite sides may be adapted to different pressures of the fluids as it may often be desired.

FIG. 2 shows a profile view of the first 2 and second 3 bulges along lines A and B, represented by a dashed and solid line, respectively.

By shaping the first 2 and second 3 bulges in way that they have different surface areas (first 4 and second 5 surface area), it is first of all possible that the flow characteristics (which have an influence on the pressure drops of the fluids) can be made different at the two sides of each of the plates 1 and hence can be made different for the two fluids involved. Furthermore, due to the different size the contact zones 4, 5 of two adjacent plates 1 (where the contact zones 4, 5 are connected by material engagement 10) it is possible to design the final heat exchanger 9 in a way that it can have a higher pressure resistance towards one fluid, as compared towards the other fluid.

Therefore it is possible to design the resulting heat exchangers 9 according to the specific requirements. In particular, the sizes (both absolute and relative) and distributions of the first 2 and second 3 bulges may be designed in such a way that specific flow rates and/or pressure drops can be obtained. At the same time the contact zones 4, 5 of the heat exchanger plates 1 can be dimensioned according to the required strength.

In the illustrated first embodiment, the surface areas of both the first bulges 2 and the second bulges 3 show an oval shape with the elongated diameter (i.e. the main axis of the ellipse) pointing substantially in the direction of the fluid flow. This way, the cross-section in the direction of the fluid flow is minimised and hence the fluid flow resistance of the fluid (and consequently the pressure loss in the fluid) can be reduced.

First experiments indicate that forming the flat tops 4 and 5 with an elliptical shape is superior to forming them with circular shapes. There is some indication that circular shapes are prone to cracks in the side walls of the first 2 and/or second 3 bulges. While the strength of the connection by material engagement 10 between neighbouring heat exchanger plates 1 depends highly on the surface areas of the flat tops 4 and 5, the load capacity of the walls depends strongly on the circumferential length and the thickness of the plate sheet. If the thickness of the plates were to be changed in order to obtain a similar strength of the walls and the connections 10, the heat exchanging effectiveness of the heat exchanger 9 would be adversely affected. Using an elliptic shape for the first 2 and/or the second 3 bulges the circumferential length can be easily increased with constant plate sheet thickness and/or surface area of the connections 10.

As a matter of completeness, it should be mentioned that according to alternative embodiments any other suitable shape for the first 2 and/or the second 3 bulges is possible as well. In particular, by using different shapes, it is likewise possible to increase the circumferential lengths without increasing the surface area of the connections 10.

In FIG. 3 a plurality of heat exchanger plates 1 that are connected to each other using connections by material engagement 10 are shown in a view from the side. The direction of the view is parallel to the lines A and B of FIG. 1. It can be seen that channels 6, 7 with two different cross-sections are formed. The larger channels 6 are formed by the heat exchanger plates 1 between the first bulges 2 with the first tops 4, showing the smaller surface areas. Of course, the connections between the (smaller) first tops 4 will yield a weaker connection as compared to the connections between the (larger) second tops 5. Furthermore, between the second bulges 3, smaller second channels 7 are formed. However, these smaller second channels 7 are suitable for higher pressurised fluid due to the stronger mechanical connections 10 between the (larger) second tops 5.

According to the embodiment of the heat exchanger plate 1 that is shown in FIGS. 1 to 3, first 2 and second 3 bulges are placed symmetrically in a rectangular grid, with first 2 and second 3 bulges on every other grid point. Thus, they are located alternating each other along a number of parallel lines, the distance between first 2 and second 3 bulges being equal and the distance between such parallel lines being equal. The channels 6, 7 that are formed for the fluids will then follow an essentially zig-zag line. In other words, the respective fluid is not forced to flow over ridges and valleys as in the herringbone pattern. Instead, it will only encounter the rounded, “pillar-like” constrictions (in form of first 2 and second 3 bulges) at the connecting points 10 between the stacked heat exchanger plates 9.

Naturally, first 2 and second 3 bulges will still cause a certain amount of variation in fluid flow rate and direction and some turbulence in the fluid. However, it is known that it is usually not desirable to eliminate turbulence completely, because usually laminar fluid flow gives poorer heat transfer rate. With the proposed pattern of bulges 2, 3 slight to moderate fluid flow rate variation in the fluid is obtained. Thus a lower pressure drop across the heat exchanger 9 per heat transfer unit is obtained for a given average fluid flow rate of the fluid. The mechanical power required to force a fluid through the heat exchanger 9 per heat transfer unit is therefore also lowered, in particular when compared to a heat exchanger with a herringbone pattern.

For improved fluid flow characteristics, the first 4 and second 5 flat top areas are presently positioned such that their longest diameters (main axis of the ellipse) substantially extend in a direction parallel to the direction of fluid flow in the heat exchanger 9. The direction of flow in the heat exchanger may be defined as the local main flow direction of the fluid, when averaged over a plurality of bulges 2, 3.

However, they could also be positioned with their longest diameter arranged with any angle relative to the direction of fluid flow in the heat exchanger 9, and may even show varying angles over the surface of the heat exchanger plates 1. Also, the sizes and/or shapes of the first top 4 and/or second top 5 areas may change over the surface of the heat exchanger plate 1, thus changing individual and/or relative flow and pressure characteristics locally.

A particular relevant embodiment for this is if the angles of the longest diameters are changing from substantially perpendicular to parallel relative to the direct connecting line between fluid inlet 11 and fluid outlet 12. Such an arrangement will assist the fluids entering through the fluid inlet 11 in distributing over the whole width of the heat exchanger plates 1, and again, will assist the fluids coming from the sides of the heat exchanger plates 1 to be directed to the fluid outlet 12.

As shown in FIG. 3, first 6 and second 7 channels, especially the respective centres of first 6 and second 7 channels, have a gap 8 with a straight, essentially undisturbed fluid flow path.

When looking at a second channel 7, for example, the fluid does not need to change its direction because of the proximity to the upper first tops 4. Still, the fluid is affected to some extent by the proximity of the left and right second tops 5. If a heat exchanger 9 with channels 7 of this type is used with a two-phased fluid, i.e. a fluid that is a mixture of both gas and liquid, the gas phase tends to flow along said gap 8 in the centre of the second channel 7. This means that the gas can flow through the heat exchanger 9 without compromising the wetting of the walls of the heat exchanger plates 1 by the liquid phase of the fluid. This provides better heat transfer. The same applies to the first channels 6 in analogy.

In some operational cases, nuclear boiling can also occur instead of surface evaporation along the walls of the heat exchanger plates 1. Such nuclear boiling will occur especially in hollows, where the fluid flow rate is significantly reduced. Such nuclear boiling will further improve the heat transfer rate.

In an alternative embodiment (not shown), the first 2 and second 3 bulges are located symmetrically in a grid, but unlike the embodiment of a heat exchanger plate 1 as shown in FIGS. 1 to 3, the grid is arranged so that the channels 6, 7 formed are parallel with the edges of the heat exchanger plate 1. This arrangement usually results in a lower pressure drop but also a lower heat transfer rate, because the tops 4, 5 obscure one another.

However, the arrangement can be modified in essentially any way. In particular, the pattern does not need to be symmetrical over the whole plate. This way, different arrangements can be used to direct the flow of fluid in the desired way and to control turbulence and pressure drop.

Furthermore, it is not necessary that the pattern of first 2 and second 3 bulges (and presumably even more different types of bulges; not shown) covers essentially the whole of the heat exchanger plate 1. The pattern can be combined with deflecting barriers and baffles, with completely flat surfaces, and also with conventional herringbone patterns if this is required for whatever reason. 

1. A plate heat exchanger, comprising at least one heat exchanger plate, wherein at least one of said exchanger plates comprises at least one section including indentations configured to be placed against corresponding indentations of a heat exchanger plate of a corresponding design, wherein the indentations include at least a first type of indentations and at least a second type of indentations, wherein said first type of indentations and said second type of indentations are of a different design.
 2. The plate type heat exchanger according to claim 1, wherein said first type of indentations and said second type of indentations are of a different size.
 3. The plate type heat exchanger according to claim 1, wherein said first type of indentations and said second type of indentations include essentially the same shape.
 4. The plate type heat exchanger according to claim 2, wherein said first type of indentations and said second type of indentations include essentially the same shape.
 5. The plate type heat exchanger according to claim 1, wherein said first type of indentations and said second type of indentations are of a different shape.
 6. The plate type heat exchanger according to claim 1, wherein the number of said first type of indentations and said second type of indentations are different.
 7. The plate type heat exchanger according to claim 5, wherein the number of said first type of indentations and said second type of indentations are different. 